In a vehicle equipped with an electric motor as a traveling drive source, the electric motor is capable of executing regenerative braking, and by collecting electric energy converted from kinetic energy of the vehicle by the regenerative braking, energy efficiency can be increased.
There are various types of vehicles that are able to execute regenerative braking. One of those is a vehicle in which front wheels and rear wheels have different regeneration capabilities, that is, a vehicle whose front wheels and rear wheels are asymmetrical with regard to regeneration. For example, there is a vehicle in which only the rear wheels can execute regenerative braking and the front wheels can execute only friction braking; or, there is a vehicle in which the front wheels and rear wheels have different transmission efficiency in regenerative braking, causing a difference in regeneration capability in such a way that the regeneration capability of the front wheels becomes smaller than that of the rear wheels.
In such a vehicle whose front wheels and rear wheels are asymmetrical with regard to regeneration, by increasing the braking force distribution to the rear wheels as much as possible within a range in which the traveling stability of the vehicle can be sufficiently ensured, it is possible to increase the amount of regeneration, thereby increasing the energy efficiency of the vehicle. Well-known technology based on such a concept has been disclosed, for example, in patent literature 1 and patent literature 2.
In technology in patent literature 1, in the diagram of the braking force distribution line between front and rear wheels, by controlling the regenerative braking force so that the total braking force line showing the sum of the fluid pressure braking force and the regenerative braking force can be located at the rear-wheel locking limit or its vicinity, it is made possible to make the best use of regenerative braking within the range other than the area where the rear wheels lock first.
Also, in technology in patent literature 2, to increase the amount of regeneration of the front and rear motors, upper-limit braking torque of the rear wheel motor is specified according to the state of the road surface (road-surface friction coefficient and the like) so that the amount of regeneration can be distributed by the front wheel motor and the friction brake. That is, when the target braking torque of the rear wheel motor becomes too much, the rear wheels tend to easily slip, and therefore, the target braking torque of the rear wheel motor is controlled so that it will not exceed the allowable maximum value according to the state of the road surface. By doing so, it is possible to distribute as much torque as possible with regard to the target reduction torque to the target braking torque of the rear wheel motor while limiting the rear wheel motor within the range in which a predetermined frictional force between the rear wheels and the road surface can be ensured.    Patent literature 1: Japanese Patent Laid-open No. Hei 9(1997)-104333    Patent literature 2: Japanese Patent Laid-open No. 2005-151633